Absorbent articles such as disposable diapers, sanitary napkins, and the like, manufactured from wood pulp have become staple items of commerce. Heretofore, these items have been primarily made from chemical pulp (e.g. wood pulp made by the sulfite process or by the kraft process). While these absorbent articles have been of good quality, the use of chemical pulp offers certain disadvantages. One major disadvantage is inherent in the chemical pulping process itself. Only about 50% of the wood entering the chemical pulping process is recovered as pulp. The remaining fraction of the wood, as well as the concentrated chemicals used in the pulping process, contribute to both atmospheric and ground water pollution unless expensive steps are taken to control plant emissions. Another disadvantage of the use of chemical pulp in absorbent articles is the relatively low bulk of the chemical pulp. (Bulk, the reciprocal of density, is a measure of the ability of wood pulp to make a product of low inherent density.) Wood pulps having high bulks make products with low densities. Since absorbency is inversely related to density, products with low densities are more absorbent on a weight basis than are products with high densities. The use of a wood pulp with higher bulk allows the manufacture of a product having relatively greater absorptive capacity on an equal weight basis than a similar product made from wood pulp with lower bulk. Alternatively, on an equal absorptive capacity basis, products made from a wood pulp with higher bulk will contain a smaller quantity of fiber than a product made from wood pulp with lower bulk.)
In more recent times, these two described disadvantages have been at least partially overcome through the use of high yield fiber. As used herein, the term "high yield fiber" denotes a wood pulp which is made by a process which allows significantly more of the entering wood to be recovered as wood pulp fiber than do the conventional sulfite or kraft pulping processes. High yield fibers are classified into numerous different types.
One of the oldest and most widespread high yield fibers is known as groundwood. It is produced by mechanically reducing the wood to fibers as by pressing the wood against a rotating stone. Groundwood, which is sometimes known by the generic term mechanical pulp, has found little application in absorbent articles such as diapers because this method of fiber separation leads to significant fiber shortening and damage before a reasonably low level of fiber bundles, i.e. shieves, is obtained.
Another form of mechanical pulp which has found somewhat greater use in absorbent products is broadly known as thermomechanical pulp. Thermomechanical pulp, which is generally attributed to the work of Asplund and his coworkers as described in U.S. Pat. No. 2,008,892 (July 23, 1935) and its progeny, involves the mechanical defibration of wood after the lignin has been softened by steaming.
Semi-chemical pulp, sometimes known as chemimechanical pulp, or semi-mechanical pulp, is a refinement of the basic thermomechanical process. Here, wood chips are given a mild chemical treatment during a heating step prior to mechanical difibration in a device such as a rotating disc defibrator. The chemical treatment is limited so as to merely soften the lignin rather than completely remove it as in conventional chemical pulping processes. Workers such as Beverage and Keough in U.S. Pat. No. 2,422,522 (June 17, 1947), Beverage, Keough and Surino in U.S. Pat. No. 2,425,024 (Aug. 5, 1947) and Asplund, Cederquist and Reinhall in U.S. Pat. No. 3,338,525 (Aug. 29, 1967) have described semi-chemical high yield fiber processes.
Also within the prior art is a semi-chemical high yield fiber process which yields a product having relatively high bulk and a relatively low shieve content. This particular process comprises the steps of preheating wood chips, treating the heated chips with a chemical solution which comprises sodium sulfite and, optionally, basic chemicals, at such a concentration as to yield pulp having a pH greater than 5.7. The chips are then mechanically defibrated to pulp with an energy consumption of less than about 600 kilowatt hours per metric of pulp produced. (Anonymous, Research Disclosures, March, 1978, p. 20.)
Generally speaking, following the pulping operation high yield fibers are formed into sheets by any of several well known wet forming processes typified by the conventional Fourdrinier process. (The operation of forming the wood pulp fibers into sheets is sometimes known as lapping.) The sheets are then usually dried with conventional equipment. For use in absorbent products such as diapers, the sheets are comminuted and the high yield fibers are formed into absorbent products known as airfelts.
The high yield fibers most preferred for use in absorbent products such as diapers are generally derived from softwoods (gymosperms). One of the problems associated with the use of these softwood high yield fibers in absorbent products heretofore has been the weakness of the sheets of pulp fibers. These sheets must be strong enough to be handled by commercial equipment during the airfelt making process, but sheets of the preferred softwood high yield fibers are generally too weak to be used as is. In fact, it is impossible to form sheets at all from some of the most preferred softwood high yield fibers.
One way the strength of the sheets has been improved has been by mechanically refining the fibers. This operation, which is known to increase strength in almost all papermaking areas, suffers from the disadvantages of increased cost and those effects which flow from the mechanical damage to fibers caused by the refining process, e.g. increased fines, lowered drainage rate, increased density, lowered absorbency. Refining is not, however, necessarily effective with all softwood high yield fibers.
Sheet strength has also been improved by adding a quantity of chemical pulp to the softwood high yield fibers. Chemical pulp, which may comprise up to 25% or more of the total pulp mixture, can optionally be refined in the wet state prior to its addition to the pulp blend. While the addition of chemical pulp does increase the strength of the wet laid sheets of softwood high yield fibers, certain adverse effects do occur. One of the most readily apparent, of course, is the increased cost of the total fiber mixture which results from the replacement of relatively low cost high yield fiber with relatively high cost chemical fiber. A second, more subtle adverse effect is the increase in wet density of the airfelt which results when chemical fibers are blended with the softwood high yield fibers.
High yield fibers are essentially non-delignified; that is, most of the lignin remains with the cellulosic fiber. This lignin contributes to the stiffness of the fiber. It has been found that these stiff fibers form airfelts having lower wet density than do conventional chemical pulp fibers. That is, if equivalent airfelts are formed from chemical pulp fibers and from stiff, non-delignified, high yield fibers, and the airfelts are compressed dry to the same initial density, the high yield fiber airfelts exhibit lower density when wet and under load than do the chemical pulp fiber airfelts.
Although Scott, in U.S. Pat. No. 2,642,359 (June 16, 1953) has suggested that the strength of a pulp sheet can be enhanced by incorporating into the fiber furnish from which the sheet is made a quantity of short fibers which tend to bind together long fibers, it is well known in the art that hardwood high yield fibers (which are generally shorter than softwood fibers) contribute to the weakness of sheets of fibers.
There are basic anatomical differences between softwoods and hardwoods. The arbitrary term softwood and hardwood designate, respectively, trees having needle or scalelike leaves and trees having broad leaves which are deciduous in temperate zones. The hardness or density of the wood is not involved. While there are differences between the chemical structures of hardwood and softwoods, the important difference, for this invention, lies in the variation in cell structure. Softwoods for the most part are made up of cells whose length is several hundred times their diameter. That is, even though barely visible to the eye, they are threadlike. Hardwoods, on the other hand, are made up of a wider variety of cell types characterized by a length to diameter ratio which may run from 1:1 to 20:1. Hardwood fiber is generally considered to be inferior to softwood fiber for certain applications. Because its ratio of length to diameter is so much smaller, the bonding between fibers is poorer because the inter-fiber crossings per fiber are fewer and the bond area of each is smaller. Consequently, a sheet is generally weaker when it contains hardwood fibers. Generally commented on the lower strength properties of hardwood sheets relative to softwood sheets in a paper entitled "Poplar Groundwood in Different Grades of Paper" delivered the EUCEPA Symposium on Mechanical Pulp held in Oslo, Norway during June, 1970, as reported in the Abstract Bulletin of the Institute of Paper Chemistry, Vol. 42, No. 3, Abstract No. 2647 (September, 1971).